Disk drives are employed as large volume data storage devices in applications with respect to computers and data processors. The data is stored on the surfaces of the memory disks in such drives. The memory disks are journalled for rotation on a disk spindle mounted on a supporting structure such as a base or a main frame of the disk drive. Data heads are used to write narrow tracks of data on the surface of data disks, that can be repeatedly read back at a later time by positioning the data heads, in what is called a track following mode, over these narrow data tracks as the disks are rotated. Data tracks on the disk surfaces are currently on the order of about 10 microns wide. To reliably read data from such a narrow pre-recorded data track, the data head must be positioned over the data track to within about a half micron of the center of the track.
A data head is flexibly secured to the end of each arm of an arm stack comprising part of a movable positioner or actuator that is mounted on the same supporting structure as the spindle which rotatably mounts the memory disks. This may be a rotary or a linear actuator. In most high performance disk drives, one head, called a dedicated head, and an adjacent disk surface, dedicated to servo code, are used to provide input to a head positioning servo for track seeking and track following purposes. The positioning servo, using signals from the dedicated head, positions the dedicated head accurately over a selected servo code track on the surface of the dedicated memory disk in a track following mode of operation, when a data track is being written in a corresponding track on a data disk surface. After data is written in the data tracks on the data disk surfaces, any mechanical displacements, that is, changes in relative position, that develop between the servo head and the data heads, results in data head offset from track center and reduces the integrity of the data which is being read. If the positioning errors are unpredictable in the presence of temperature variations for example, then there is little that can be done towards preserving the integrity of the data which is being read. If, on the other hand, positioning errors develop that are predictable, then it is possible to characterize and integrate correctional feedback information to the servo system, as will be explained at a later point.
Typically, disk drive supporting structures are made as rigid and stiff as possible for the purpose of minimizing head offset from the track center of a pre-written data track on a disk in a subsequent read operation. The particular material used for each of the various parts within the drive is chosen for optimum functionality and cost. But because of this, some significant: drive performance problems often arise.
First, when stiff, rigid parts are fastened to each other, they are no longer free to change dimension independently of one another. In the fabrication of a disk drive, parts are assembled at very nearly the same temperature. But when the drive is operated, heat is developed and temperature gradients occur. These temperature gradients cause the different parts of the drive to change in dimension, which may create enough stress at the fastened locations to force minute unpredictable slip in the relative position of the parts.
Second, when structural parts of the disk drive are made from different materials, the thermal coefficients of expansion also tend to be different. When these rigid parts are fastened together, as in a disk drive supporting structure and exposed to temperature extremes, as often encountered in shipment, the stresses that develop at the connection areas can easily cause the connection to permanently slip. Once again, this slip is unpredictable as to exactly when and how much will occur.
In practice, disk drives have been thermally cycled prior to writing data tracks on the disk surfaces in an effort to somewhat relieve connection area stresses and thereby minimize thermally induced positioning errors. While this process improves the probability of long term positioning accuracy of the drives, it does not solve the problem and some unacceptable percentage of the drives that are thermally cycled will still be prone to positioning problems. The time and the cost of thermally cycling production quantities of disk drives is also undesirable.